1. Field of Invention
This invention relates to telecommunications systems. Specifically, the present invention relates to a system and method for digitally encoding and decoding speech.
2. Description of the Related Art
Transmission of speech by digital techniques has become widespread for telephony, voice email, and other applications. This, in turn, has created interest in improvements in speech processing techniques. One area in which improvements are needed is that of spectral enhancement, in particular, low frequency spectral enhancement. In systems where much of the low frequency content has been lost, energy may be removed from the fundamental pitch harmonic of the voice signal, causing the voice to sound "tinny." Loss of low frequency content may be due to the acoustic features of the equipment being used, the analog electronics or the transmission path characteristics of the system, or the effects of digital processing of the voice signal.
In equipment such as a phone, the acoustic features are defined by the phone design (plastics, microphone placement), the way a user holds the phone, and the environment that a user is in. The shape of the plastics may create an acoustic null at certain frequencies. The way a user holds the phone affects the acoustic response because the user may, for example, not talk directly into the microphone. The user's environment affects the acoustic frequency response by altering the characteristics of a signal transmitted through the environment. For example, when a hands-free phone is used inside a vehicle, acoustic reflections bouncing around inside the vehicle combine together and may cause the voice to sound tinny.
In a phone, the microphone transforms the acoustic signal into an electrical signal. The electrical signal is processed by analog electronics, which filters the signal so that the low frequencies may be attenuated. If the electrical signal carrying voice information is passed through an analog transmission medium, such as a twisted wire pair or coaxial cable in the telephone network, the frequency content of the voice signal may be further affected.
In the digital domain, the use of noise suppression may cause the voice to sound tinny. Noise suppression generally serves the purpose of improving the overall quality of the desired audio signal by filtering environmental background noise from the desired speech signal. Noise suppression is particularly important in environments having high levels of ambient background noise, such as an aircraft, a moving vehicle, or a noisy factory. Noise suppression may cause the voice to sound tinny because the noise sought to be suppressed is concentrated in the low frequencies.
Hence, a need exists in the art for an improved system and method for enhancing the low frequency spectral content of digitized voiced speech.